title: Photoinactivation of photosystem II: is there more than one way to skin a cat?
abstract: We briefly review the main mechanisms proposed for photodamage to photosystem II (PSII), at the donor and acceptor sides, and then discuss the mechanism whereby filamentous cyanobacteria inhabiting biological sand crusts such as Microcoleus sp. are able to avoid serious damage to their photosynthetic machinery. We show that the decline in fluorescence following exposure to excess light does not reflect a reduction in PSII activity but rather the activation of a non-radiative charge recombination in PSII. Furthermore, we show that the difference in the thermoluminescent peak temperature intensities in these organisms, in the presence and absence of inhibitors such as dichlorophenyl-dimethylurea (DCMU), is smaller than observed in model organisms suggesting that the redox gap between Q(A)⁻ and P₆₈₀+ is smaller. On the basis of these data, we propose that this could enable an alternative, pheophytin-independent recombination, thereby minimizing the damaging ¹O₂ production associated with radiative recombination.
authors.affiliation: Department of Biological Chemistry, The Hebrew University of Jerusalem 91904, Edmond J. Safra campus, Givat Ram, Jerusalem, Israel. ohad@vms.huji.ac.il
authors.forename: Itzhak
authors.initials: I
authors.lastname: Ohad
authors.forename: Alex
authors.initials: A
authors.lastname: Berg
authors.forename: Simon M
authors.initials: SM
authors.lastname: Berkowicz
authors.forename: Aaron
authors.initials: A
authors.lastname: Kaplan
authors.forename: Nir
authors.initials: N
authors.lastname: Keren
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D008027:Light; D055668:Photochemical Processes / Q000528:radiation effects*; D045332:Photosystem II Protein Complex / Q000378:metabolism*; D026082:Singlet Oxygen / Q000378:metabolism; D014026:Nicotiana / Q000378:metabolism / Q000528:radiation effects
chemical_list: D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D026082:Singlet Oxygen